codereview: accept Github PRs Issue #18517 golang/go GitHub

I propose we start accepting pull requests by automatically converting them into Gerrit CLs ("change lists", same as a PR but different terminology). Reviews would still happen on Gerrit and the bot would update the PR of activity on Gerrit. Gerrit is still where we'd run trybots and push the "Merge" button. We would never merge on Github. Gerrit would remain the upstream source-of truth.